We all like to think of ourselves as totally unique, independent individuals, in charge of our own destinies. But new research has found evidence that our behaviour, and maybe even our personalities, could be influenced by something totally unexpected - our immune systems.
Researchers have shown that by switching off just one immune molecule in mice, they can change the way the animals behave and interact with each other - which suggests the immune system may play a role in conditions such as autism-spectrum disorder or schizophrenia.
Before we get too carried away, this is early research that's only been conducted in rodents for now. But the researchers from the University of Virginia School of Medicine were able to clearly show that by simply changing the way the immune system responds to pathogens, they could trigger antisocial behaviour in mice.
Restoring the molecule returned the mouse personalities to normal.
"It's crazy, but maybe we are just multicellular battlefields for two ancient forces: pathogens and the immune system," said lead researcher, Jonathan Kipnis. "Part of our personality may actually be dictated by the immune system."
The molecule in question is called interferon gamma, and it's usually released by the immune system when it comes into contact with a pathogen, such as a virus or bacteria.
This type of immune response is part of the adaptive immune system, which learns to keep an eye out for nasty germs - and up until last year it was thought to be isolated from the brain as a result of the blood-brain barrier.
But that all changed in 2015, when Kipnis and his team discovered for the first time that meningeal vessels directly link the brain to the lymphatic system, which means that the brain and the immune system can directly interact, something that was previously thought to be impossible.
"The brain and the adaptive immune system were thought to be isolated from each other, and any immune activity in the brain was perceived as sign of a pathology," explained Kipnis. "And now, not only are we showing that they are closely interacting, but some of our behaviour traits might have evolved because of our immune response to pathogens."
This link between the immune response and the brain could explain a lot - for years, scientists had suspected that conditions such as depression, autism, and schizophrenia might somehow be triggered by the immune system, and the research offered a possible explanation for how that could be happening.
But Kipnis and his team took things one step further, and hypothesised that if the pathogens and the immune system could be linked to certain social conditions, then it could also be influencing our broader social interactions and personality.
As the University of Virginia explains:
"The relationship between people and pathogens, the researchers suggest, could have directly affected the development of our social behaviour, allowing us to engage in the social interactions necessary for the survival of the species while developing ways for our immune systems to protect us from the diseases that accompany those interactions."
From an evolutionary point of view, this makes sense, because social behaviour would be in the interest of pathogens to help allow them to spread. And for us, the social behaviour leads to reproduction and the propagation of the species, so it's a win/win.
To investigate whether this could be the case, in the latest study, the researchers switched off the immune molecule interferon gamma in mice, flies, zebrafish, and rats. Because this molecule runs and tells the rest of the immune system when germs are about, they were testing what would happen when that interaction was shut down.
In all species, they showed that interferon gamma was essential to normal social interaction.
They found that blocking the molecule in mice caused the animals' brains to become overly connected, making the mice less willing to interact with others.
You can see a gif representing the extra connections forming in the mice's brains below (the normal brain has similar connections, but fewer of them):
Reinstating the molecule restored their brains to normal, and saw them resume social activities, showing a clear link between the immune system and behaviour - in mice, at least.
The team has published their research in Nature, and concludes that the immune molecule plays a "profound role in maintaining proper social function".
What this means for humans remains to be seen, and more research is now needed to investigate whether interferon gamma plays the same role in people's social behaviour.
It's still very early days, but having some insight into how germs and the immune system could control our behaviour opens up a lot of potential to further understand why we act the way we do, and why things occasionally go wrong. It could even lead to new treatments for people with social disorders one day.
"Immune molecules are actually defining how the brain is functioning. So, what is the overall impact of the immune system on our brain development and function?" said Kipnis. "I think the philosophical aspects of this work are very interesting, but it also has potentially very important clinical implications."